One of the most important characteristics of a liquid chemical is its viscosity. In fact, special chemical components called “rheology modifiers” are commonly added to such chemical compounds in order to maintain their viscosity within a desired range or to exhibit a desired viscosity profile. One particularly desired viscosity profile that can be obtained through the use of rheology modifiers is a “pseudoplastic” or “shear thinning” type viscosity profile.
In a shear thinning profile the liquid composition exhibits high viscosity at low shear conditions, and lower viscosity at high shear conditions. One readily identifiable chemical compound with a shear thinning viscosity profile is house paint. When paint is subjected to high shear (for example by agitating or stirring it with a brush), the paint “thins” and this decrease in the viscosity allows the brush to be moved through the paint. Correspondingly, under low shear conditions (e.g., when the movement or agitation of the brush stops), the paint thickens and adheres to the bristles of the brush with minimal dripping. When the paint is applied to the surface to be painted with either a brush or a spray nozzle the paint is again subjected to shearing forces that cause the paint to thin again and flow off the brush onto the painted surface. As the paint lies on the surface, it is being subjected to only miniscule amounts of shear (by the force of gravity) so that the paint thickens, thus keeping it in place and preventing undesirable dripping or sagging until the paint dries.
As with paints, so also other liquid chemicals and polymers are formulated to have shear thinning viscosity profiles. One such example of a rheology control agent is fumed silica, which is prepared from SiCl4 in a flame hydrolysis process. Despite the advantages of fumed silica, product formulators have cited problems with both hydrophobic and hydrophilic forms of fumed silica. For example, because of its moisture-absorbing silanol groups, hydrophilic fumed silica tends to cause problems in electronic adhesives or coating applications because the increased water concentration decreases electrical resistance. Additionally, in coating applications, this moisture, introduced by the hydrophilic fumed silica, can accelerate corrosion of the coated substrate. Also, the shear-thinning efficiency of hydrophilic fumed silica is often inadequate, which is thought to result from adsorption of the liquid onto the fumed silica surface, preventing silica aggregation and thus shear-thinning.
It has been suggested that many of the problems associated with hydrophilic fumed silicas can be avoided by treating the fumed silica with organotrialkoxysilanes. See Shirono, et al., J. Colloid and Interface Science 239, 555–62 (2001). Alternatively, fumed silica products treated with polydimethylsiloxane are also available, such as the Aerosil® R202 fumed silica from Degussa Corporation, Parsippany, N.J.
However, for many product applications use of surface modified fumed silica is impracticable, because fumed silica is simply too expensive. Also fumed silicas can be cumbersome to process during manufacture. In fact, when adding fumed silica to a liquid polymer composition, even high shear blending is insufficient to disperse the fumed silica particles. It is often necessary to add a process step where the fumed silica-polymer mixture must be passed through a media mill or a three-roll mill in order to sufficiently disperse it. Dispersing fumed silica often necessitates having to make a master batch of the combined fumed silica and liquid to build enough viscosity to increase shear to help dispersion. Furthermore, there is the danger of overdispersing fumed silica which breaks the silica aggregates, ruining the shear-thinning properties. Thus, eliminating the need for these extra process steps as well as avoiding the danger of overdispersing the silica would be highly desirable to product formulators.
Given the foregoing, there is a continuing need for a rheology modifier that has a number of performance characteristics, such as: 1) imparts a strong shear thinning profile to chemical compositions to which it is added; 2) disperses easily into liquids and slurries, eliminating the need for the additional dispersing steps; 3) eliminates the danger of overdispersion of the silica; and finally 4) is sufficiently inexpensive to be used in a wide range of applications.